1. Field of the Invention
This invention relates to the use of alternating current (AC) dimmer switches with non-incandescent electrical loads.
2. Prior Art
The first electrical dimmer switches used resistive elements to reduce power flow in alternating current (AC) waveforms to the load, such as U.S. Pat. No. 1,496,449 to Briggs (1924). Due to the power loss in these switches, modern dimmer switches now relay on components called SCRs (Silicon Controlled Rectifiers) and TRIACs (TRIodes for Alternating Current) to control the output waveform. The most common of these systems involves switching of the TRIAC to the conductive state at some point on the AC waveform so that only a fraction of the complete waveform reaches the load, as seen in U.S. Pat. No. 4,287,468 to Sherman (1981). This altered or “chopped” waveform is very effective at dimming incandescent loads, but produces unwanted behavior in alternative light sources.
A solution has been developed to replace the dimmer switch with a modified dimmer switch, adding circuitry to operate with a specific load such as a fluorescent light (U.S. Pat. No. 3,935,505 to Spiteri (1976)), or an LED light source (U.S. Pat. No. 7,327,098 to Chen (2008)). This adds cost as it requires the addition of a modified dimmer switch, which can be a significant factor in a retrofit situation, based on the intent of using existing wiring and switches. A new development in LED technology for a semiconductor based light emitting device which can be driven directly by an AC power source (such as US patent application US2008/0087902) highlights the need for a cost effective method of retrofitting fixtures without changing any of the existing wiring and switching infrastructure, such as legacy dimmer switches.
Previous solutions have been developed to add features to an incandescent base to enhance operation of said incandescent bulb without changing existing wiring or switches, such as U.S. Pat. No. 3,916,249 to Ackermann (1975). Expanding this concept to non-incandescent loads, U.S. Pat. No. 7,102,902 to Brown/Lodhie (2006) details a device that can sit between the dimmer switch and LED light source (specifically as part of the LED lamp base or fixture) to enhance compatibility with conventional TRIAC dimmers. The Brown/Lodhie invention provides a method of creating a sufficient load near the zero point on the AC waveform to allow the TRIAC to switch off properly, enabling the dimmer switch to output a waveform similar to how it would with an incandescent load, without wasting power as the load is only present near the zero point on the AC waveform. While this is successful at shunting leakage current away from the load (so that it is not lit or energized when the dimmer switch is off and at allowing the TRIAC dimmer to shut off at the proper times, this design suffers from two disadvantages:    A) It does not provide a large, linear range of dimming as many non-incandescent light sources (such as LEDs) have minimum turn-on voltages, not allowing the light source to conduct (and hence emit light) below a certain threshold on the dimmer switch, frequently more than halfway up to a 100% brightness setting.    B) It does not provide smooth light output in a “dimmed” state as non-incandescent light sources (such as LEDs) are very responsive to “choppy” AC waveforms such as the output of a TRIAC dimmer, resulting in a flicker at certain light output levels.
A solution is needed to modify the incoming “chopped” AC waveform from a commercially available dimmer switch, which is based on duty cycle as a percentage of total sine wave, into a proportional amplitude varied waveform (such as what used to be output by a resistive dimmer) allowing for smooth, linear dimming over a large range of user selectable light output levels with no flicker or interference wherein inconsequential power is used or wasted in the conversion.